It is known to propel a boat or other water craft using a pump jet apparatus mounted to the hull, with the powerhead being placed inside (inboard) the hull. The drive shaft of the pump jet apparatus is coupled to the output shaft of the inboard motor. The impeller is mounted on the drive shaft and housed in a jet propulsion pipe or water tunnel.
To facilitate use of pump jet-propelled boats in shallow water, it is known to mount the pump jet at an elevation such that the pump jet does not project below the bottom of the boat hull. This can be accomplished, for example, by installing a duct in the stern of the boat, the duct being arranged to connect one or more inlet holes formed in the bottom of the hull with an outlet hole formed in the transom. The pump jet is then installed outside the hull in a position such that the pump jet inlet is in flow communication with the duct outlet at the transom. Such a system is shown in Australian Patent Specification No. 262306, published in 1963. Alternatively, the pump jet can be installed inside the duct built into the hull, as shown in U.S. Pat. No. 5,181,868.
Alternatively, a water tunnel can be formed in the stern of the boat hull, the water tunnel having a height which gradually increases from the hull bottom at a point in front of the transom to a maximum height at the transom. In one type of design, the pump jet is placed so that its inlet is in flow communication with the outlet of the water tunnel. One example of this type of system is shown in Italian Publication No. 724662.
In another type of design, part of the pump jet apparatus is installed inside the hull while the remaining part penetrates the transom and extends to the rear of the hull. An inlet housing of the pump jet has a horizontal opening and an inclined water tunnel for guiding water to the impeller. The horizontal opening of the inlet housing is mounted in a hole in the bottom or near the bottom of the hull. A similar design is disclosed in Swiss Patent No. 481788.
In many pump jet units powered by inboard engines, the drive shafts and pump mountings (which must penetrate the hull) are placed below the waterline. Such a mounting system has the disadvantage that various gaskets and seals are required to ensure the integrity of the installation. Leakage at any of the mounting and shafting locations can be disastrous.
In addition, in the event that the inlet to the pump jet becomes clogged with debris, permanent mounting of the pump jet makes it difficult to clean out the debris from the inlet. In some instances, removal of the ingested or entangled debris requires that the boat user enter the water or even that the boat be lifted out of the water.
Thus, an arrangement for mounting a pump jet in flow communication with a water tunnel wherein the hull penetrations are located above the waterline provides an advantage. Further, it would be advantageous to mount the pump jet in such a way that it can be removed from its position in flow communication with the water tunnel and into a position which allows easy access to any debris clogging the pump jet inlet.